The invention relates to a method for producing puzzolanic or hydraulic grinding additives for the cement industry from basic oxidic slags.
From EP 666 930 B1, a method has already been known, in which steel slags are reduced using pig iron and, in particular, the carbon content. present in the pig iron, thus causing, on the one hand, the pig iron bath to be refined and, at the same time, the iron oxide from the steel slag to be reduced to iron and get into the metal bath. Oxidic slags and, in particular steelworks slags, depending on their metal oxide contents and their basicities, have a more or less high viscosity, thus calling for an operation at comparatively high temperatures in order to maintain an accordingly highly liquid slag. If such slags are, moreover, adapted in view of cement-technologically interesting compositions by suitable corrective substances, this will frequently result in slags tending to intensified foaming at usual temperatures on account of the intense CO formation from the carbon of the bath. If foams occurring in that manner are stable, this will give rise to a reduced mass transfer between metal and slag and hence a clearly lowered reduction rate, thus substantially extending the treatment time. If, at the same time, it is operated with a relatively high carbon content of the metal bath, this may provoke particularly vigorous reactions in the interface, which may cause heavy foaming and even undesired slag spittings.
The invention aims to lower the initial reaction rate and shorten the treatment time in the processing of such basic oxidic slags while, at the same time, ensuring the safe and rapid reaction and, in particular, reduction of metal oxides in the slag at a low temperature level. To solve this object, the method according to the invention essentially consists in that the basicities of the liquid slags are adjusted to values lying 0.1 to 0.5 below the basicity value (CaO/SiO2) of the target slag prior to said reduction by adding acidic additives such as, e.g., quartz sand and/or blast furnace slag and/or SiO2-containing corrective substances. By lowering the basicities of the liquid slags to relatively low values and, in particular, lower values than appear desirable for the subsequent utilization in cement technology, a slag that is highly liquid even at lower temperatures and exhibits only a slight tendency to foaming can be immediately obtained. The decrease of the basicity and the thus connected lowering of the viscosity as a function of the temperature level lead to a clearly higher reduction rate and hence a more rapid reaction during which, in particular, iron and manganese oxides are rapidly reduced to metallic iron and metallic manganese, respectively. Likewise, oxides of chromium, nickel, vanadium and other metals are reduced. With the metal oxide content decreasing, also the conversion of the carbon contained in the bath to carbon monoxide and carbon dioxide naturally goes down, thus substantially reducing the risk of slag foaming. If appropriate amounts of Al2O3 are added, the viscosity is additionally lowered, and a smaller decrease of the basicity will do in order to obtain the desired reaction rate.
Within the context of the method according to the invention, it is subsequently proceeded in an advantageous manner in that the basicities of the slags at the end, or near the end, of the reduction phase are adjusted to the desired target basicity of between 1.1. and 1.5. On account of the diminished carbon conversion and the thus declined tendency to foaming, it is, therefore, feasible to adjust the target basicity sought for cement-technological reasons already towards the end of the reduction, it being feasible to effect the respective addition in the converter in which the reduction of the steel slags was performed.
In order to adjust a preferred cement-technological composition of the slags, also an increase in the Al2O3 content is usually desirable in addition to the adjustment of a target basicity to values of between 1.1. and 1.5, wherein bauxite may, for instance, be added for that purpose. In order to ensure a rapid conversion in the reduction phase, it will be of advantage if also the Al2O3-containing corrective substances are added already at the beginning of treatment, it being advantageously proceeded such that Al2O3-containing corrective substances such as, e.g., bauxite at least partially are added prior to the reduction of the slags. By adding a partial amount of the additives required for the adjustment of the Al2O3-content prior to reduction, reliable blending within the slag at a simultaneous decrease of the slag viscosity is safeguarded during the reduction procedure, said partial amount preferably ranging between one half and three quarters of the required addition of Al2O3-containing additives.
In a particularly simple manner, the target basicity may be adjusted by adding burnt lime and/or CaO-containing corrective substances.
The lowering of the basicity to values near the neutral point prior to reduction also brings about particular advantages in respect to the service life of a nonbasic refractory lining.
Within the context of the method according to the invention, it is advantageously proceeded such that the target basicity is approached from a point of time at which the metal oxide content and, in particular, the sum of the iron oxide, manganese oxide, chromium oxide, nickel oxide and vanadium oxide contents of the slag falls below 3.5 wt.-%. From a decrease of the metal oxide content to values of below 3.5 wt.-%, foaming is already. largely avoided because of the substantially diminished conversion of the carbon contained in the bath as pointed out above, so that it is already from that point of time that the additives required for approaching the target basicity and the cement-technological composition may be added without adversely affecting the method.
In the following, the invention will be explained in more detail by way of an exemplary embodiment.